1. Field of the Invention
The present invention relates to a process for the preparation of optically active citronellal, which is also referred to as chiral citronellal, by the asymmetric hydrogenation of neral [E-3,7-dimethyloctadien-2,6-al] or geranial [Z-3,7-dimethyloctadien-2,6-al] which are achiral isomeric constituents of citral.
2. Description of the Prior Art
Chiral citronellal is a valuable intermediate in organic synthesis; in particular, d-citronellal is used for the preparation of (-) (1S)-menthol, hereafter simply 1-menthol, by a process which employs the cyclization of d-citronellal into (-) (1S)-isopulegol under the influence of a proton catalyst, or by a thermal route, followed by the hydrogenation of the isopulegol into 1-menthol [cf., J. C. Leffingwell and R. E. Shackelford, Cosmetics and Perfumery, 89, 70-78 (1974)].
The d-citronellal used for the synthesis of 1-menthol is obtained from various essential oils largely comprised of d-citronellal and, in particular, from citronella oil. The use of chiral citronellal of natural origin is not entirely satisfactory insofar as, because of the fluctuation in the prices of natural products, it periodically happens that the price of d-citronellal is higher than that of natural menthol. It is therefore of importance to industry to have available a source of chiral citronellal which has a relatively stable price and leads to a synthetic 1-menthol having a lower cost price than that of natural menthol.
The synthetic achiral citronellal obtained via the hydrogenation of citral [a mixture of neral and geranial] could constitute a valuable source of chiral citronellal and especially of d-citronellal; however, no industrial process exists for resolving racemic citronellal into its enantiomers, with the result that industry does not have any process available which makes it possible to obtain chiral citronellals by a synthetic route.